One such milling fixture for stripping road surfacing has a roller-type milling cutter provided with a rotary drive. Said roller-type milling cutter is mounted on a road surface-handling machine by means of a height control device. Said roller-type milling cutter is held in a bearing frame. Said bearing frame can be elevated and lowered with respect to the road surface-handling machine by means of a hydraulic actuator, said height control device signalling the hydraulic actuator such that the roller-type milling cutter strips a defined thickness layer from the road surfacing. Said height control device has a mechanical sensor for sensing the level height of the bearing frame with respect to the unmachined road surfacing. This mechanical sensor in prior art height control devices constitutes, for example, a sensing wheel with height adjustment. Although milling cutters when "as new", i.e. when the roller-type milling cutter is unworn , permit achieving good accuracies when using such a prior art height control device for guiding the roller-type milling cutter with respect to the unmachined road surfacing so that the layer thickness stripped from the road surfacing remains essentially constant, the resulting wear of the roller-type milling cutter in the course of operation causes a reduction in the actual cutting diameter of the milling cutter. Said prior art height control device can only be made to compensate for such wear by readjusting the mechanical sensor manually. A further problem of said prior art height control device is that the mechanical sensor for sensing the road surfacing can easily be damaged under the rough conditions which normally exist in road repair work.
It is generally known that gauging can also be done with non-mechanical proximity-type sensors. Ultrasonic sensors have also been in use in the construction equipment sector for some time. However, the application of ultrasonic gauging devices in the construction equipment sector is limited to instances in which highly accurate control is not a mandatory requirement. The reason for this limitation in the scope of application is that the sensing accuracy of an ultrasonic gauging device is greatly influenced by the temperature, flow and humidity of the ambient air, by the operating voltage, by the momentary atmospheric pressure and by similar influencing factors, at least when not provided with relatively complicated means of compensation. Since the percentual overall error accumulated under worst-case conditions from these influencing factors is of a magnitude similar to that of the cutting depth to be maintained relative to the distance of an ultrasonic gauging device away from a road surface, a height control device for milling fixtures used in stripping a road surfacing exists only in mechanical form, not incorporating ultrasonic gauging.